I had this problem and it was driving me mad. Looked through this forum and someone mentioned that they changed the setting on their LG TV and it was much better. I have a Samsung smart TV so looked at the setting in the menu, Sound, Additional settings. There is an option titled "HDMI Audio Format" and it was set to "Bitstream". Changed this to "PCM" and the problem is now resolved.
I have recently had new smart tv and this issue has shown itself up. All the other devices I connect are like half volume level compared to the V6. Xbox, Switch and the in built smart apps all ok at the same level. Switch to the V6 and we have to double the volume.
I will have a look at the advanced sound settings mentioned but don't see why it should have to be changed if everything else is fine.